The female moa, the world’s tallest bird that ever lived, could stand as tall as 12 feet — up to 150 percent as tall as her mate. Indeed, the size difference is so great that, in years gone by, fossil hunters wrongly placed male and female skeletons into different species. Researchers from the Zoological Society of London (ZSL) have investigated to find out why the females are so much bigger than their mates, and they have published their findings Wednesday in the journal Proceedings of the Royal Society B.
In birds of prey, females are well-known to be larger than the males — sometimes significantly so. Because they are powerful birds evolved to hunt, the females may be larger so that they can take different prey and not have to compete directly with their consorts for food. It’s also possible that a female would be better able to defend herself from a testosterone charged male if she was bigger.
However, despite the humongous hulking female moas that might reasonably send a shiver of terror down a strong man’s spine, these recording-setting tallest birds aren’t predators. They were peaceful, flightless herbivores that lived in New Zealand up to around 1300 A.D. Maori hunters then arrived on the islands, quickly hunting the known nine species to extinction, possibly in as little as one brief century.
The ZSL study compared the New Zealand giant moa Dinornis with other flightless birds that still survive today, including ostriches and emus, as well as smaller related birds like the tinamous. In other ratites like emus, the female is again taller than the male.
They concluded that the lack of competing large herbivores in New Zealand is what allowed the giant moas to keep evolving to larger and larger sizes. Lead author Dr. Samuel Turvey explained, “A lack of large land mammals such as elephants, bison and antelope allowed New Zealand’s birds to grow in size and fill these empty large herbivore niches.”
The females, already stockier to begin with to accommodate the large eggs they would need to lay, ultimately grew much bigger than the males as “an odd side-effect of the scaling up of overall body size.”
Alas, because of overhunting, only fossils remain of the various moa species. Birders missed observing the world’s tallest bird, the female moa, by less than a thousand years.
[historical photo of fossil hunter Sir Richard Owens with female moa courtesy Zoological Society of London (ZSL)]
[photo predatory fossil bird skull by Elaine Radford]